Serial-Link Receiver Using Time-Interleaved Discrete Time Gain

ABSTRACT

A serial receiver combines continuous-time equalization, analog interleaving, and discrete-time gain for rapid, efficient data reception and quantization of a serial, continuous-time signal. A continuous-time equalizer equalizes a received signal. A number N of time-interleaved analog samplers sample the equalized continuous-time signal to provide N streams of analog samples transitioning at rate reduced by 1/N relative to the received signal. A set of N discrete-time variable-gain amplifiers amplify respective streams of analog samples. A quantizer then quantizes the amplified streams of analog samples to produce a digital signal.

BACKGROUND

Wired communication refers to the transmission of data over a wire-basedcommunication technology. Receiving such data, particularly at high datarates, requires sensitive, linear analog amplifiers. An amplifier islinear if its output is a linear function of its input, which means inpart that signal components of different frequencies receive the samelevel of amplification. Linearity is difficult to obtain in practice andis a main factor limiting the speed performance of analog receivers.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is illustrated by way of example, and not byway of limitation, in the figures of the accompanying drawings and inwhich like reference numerals refer to similar elements and in which:

FIG. 1 depicts a serial receiver 100 that uses a combination oftime-interleaving and discrete-time gain for rapid, efficient datareception and quantization of a serial, continuous-time signal Data on alike-identified single-ended or differential input node.

FIG. 2A details an embodiment of discrete-time variable-gain amplifier115(1), one of sixteen such amplifiers 115[16:1] introduced in FIG. 1.

FIG. 2B is a timing diagram 225 illustrating the operation ofdiscrete-time variable-gain amplifier 115(1) of FIG. 2A.

DETAILED DESCRIPTION

FIG. 1 depicts a serial receiver 100 that uses a combination ofcontinuous-time equalization, analog interleaving, and discrete-timegain for rapid, efficient data reception and quantization of a serial,continuous-time input signal Data on a like-identified single-ended ordifferential input node. Signal Data is conveyed at a relatively highsymbol rate, 56 GBd (56 billion symbols per second) as of this writing.A continuous-time equalizer 105 equalizes signal Data to provide anequalized continuous-time signal Deq that operates at the same symbolrate. In this context, a “continuous-time” signal is one that iscontinuous in time, and a “continuous-time” equalizer is one that isalso continuous in time, e.g. it does not use any clocking and operatesover a range of frequencies. Suitable continuous-time equalizers arewell known to those of skill in the art so a detailed discussion isomitted.

A set of sixteen time-interleaved analog samplers 110[16:1] samplesequalized continuous-time signal Deq using sixteen differently phasedclock signals Clk1 through Clk16 to obtain sixteen respective series ofanalog samples Da1 through Da16. Timing circuitry 112 derives clocksignals Clk1 through Clk16 from a reference clock signal Clksynchronized to signal Data. Each analog sample expressed in series Da1through Da16 is a voltage level corresponding to one symbol time ofequalized continuous-time signal Deq. Time interleaving samplers110[16:1] reduces the sample rate by a factor of sixteen, and thusrelaxes time constraints on downstream circuitry and increases overallgain bandwidth. In this embodiment in which signal Data is conveyed at56 GBd, for example, analog sample streams Da1 through Da16 are eachconveyed at about 3.5 GBd. The number of time-interleaved analogsamplers can be generalized to N, which may be more or fewer thansixteen in other embodiments. Analog samplers, such as so-called“track-and-hold amplifiers,” are well known to those of skill in theart.

Each of the sixteen time-interleaved analog samplers 110[16:1] feeds acorresponding one of sixteen discrete-time variable-gain amplifiers115[16:1]. Amplification through discrete-time variable-gain amplifiers115[16:1] is linear relative to continuous-time variable-gain amplifiersand contributes to the overall linearity of receiver 100. Consideringequalized, continuous-time signal Da1, for example, one of discrete-timevariable-gain amplifiers 115[16:1] provides linear amplification of thatseries of analog symbols to produce an amplified series of analogsamples Da1(k). Timing circuitry 112 controls the timing ofdiscrete-time variable-gain amplifiers 115[16:1] by issuing sixteen setsof reset and evaluation signals Rst #/Eval #, examples of which aredetailed below in connection with FIGS. 2A and 2B. The remaining fifteenamplifiers 115[16:1] similarly amplify signals Da2-Da16 to produceamplified series of analog samples Da2(k) through Da16(k). In general, asignal xn(k) represents a sample from the nth instance of n=16:0constituent amplifier 115[16:1] at a discrete time k. The voltage gainprovided by discrete-time variable-gain amplifiers 115[16:1] in thediscrete-time domain reduces the need for continuous-time amplificationat earlier stages. Reduced continuous-time amplification allows CTE 105and analog samplers 110[16:1] to operate at lower voltage levels, andthus in more linear amplification ranges. Finally, a set of sixteenconventional quantizers 120[16:1] coupled to the outputs ofdiscrete-time variable-gain amplifiers 115[16:1] quantizes analog,discrete-time signals Da1(k) through Da16(k) to produce sixteen seriesof digital signals D1(k)-D16(k) that collectively convey the digitalvalues represented by the original received continuous-time signal Data.

FIG. 2A details an embodiment of discrete-time variable-gain amplifier115(1), one of sixteen such amplifiers 115[16:1] introduced in FIG. 1.Amplifier 115(1) is a differential amplifier with a gain stage thatemploys a pair of NMOS transistors M1 and M2 with control terminals(gates) connected to complementary input nodes to receive differentialanalog signal Da1. The sources (current-handling terminals) oftransistors M1 and M2 are coupled to a lower power-supply terminal Vssvia respective current sources 200 and to one another via a variableresistance 205. The drains (also current-handling terminals) oftransistors M1 and M2 are selectively coupled to complementary outputnodes Da1(k) via ganged switching elements 210, both controlled byevaluation signal Eval1, which can be of transistors (not shown). Outputnodes Da1(k) are also coupled to upper power-supply terminal Vdd via apair of capacitors 215 and ganged switching elements 220 thatselectively discharge capacitors 215 responsive to reset signal Rst1.

FIG. 2B is a timing diagram 225 illustrating the operation ofdiscrete-time variable-gain amplifier 115(1) of FIG. 2A. The trace forsignal Da1 represents the analog voltage difference between the gates oftransistors M1 and M2, which voltage difference can be positive ornegative depending upon the value expressed by the symbol underconsideration. The trace for signal Da1(k) represents the positive ornegative voltage difference between differential output nodes Da1(k).

Signals Rst1 and Eval1, both digital, are low from time T0 to T1 to openswitching elements 210 and 210. Output nodes Da1(k) thus float atvoltages that are a function of a prior symbol. Reset signal Rst1 isasserted at time T1 to close switching elements 220 to dischargecapacitors 215. Both nodes Da1(k) are thus set to the high supplyvoltage Vdd and their differential value to zero. An evaluation stagebegins at time T2 when reset signal Rst1 returns low and evaluationsignal Eval1 is asserted. Switching elements 220 open and switchingelements 210 close. The relative voltages across capacitors 220 thuschange as a function of the currents through transistors M1 and M2, andthus the differential voltage across the gates of transistors M1 and M2.The voltage across each capacitor 215, and thus across output nodesDa1(k), changes linearly responsive to the constant currents pulled bycurrent sources 200. Beginning evaluation with output nodes Da1(k) atsupply voltage Vdd maximizes signal headroom and amplitude to furthercontribute to linearity. Source degeneration established by resistor 205also contributes by reducing the impact of the input transconductancesof transistors M1 and M2 on the gain of amplifier 115(1). Resistance 205can be tuned to establish a desired level of source degeneration and tomatch and calibrate amplifier 115(1) with respect to the othervariable-gain amplifiers 115[16:2].

The evaluation stage ends at time T3 when evaluation signal Eval1 isdeserted, leaving a measure of symbol magnitude stored as a voltageacross nodes Da1(k). Quantizer 120(1) quantizes the analog value ofsignal Da1(k) between times T3 and T4 to produce a digital value D1(k)(FIG. 1). Reset signal Rst1 is once again asserted in anticipation ofthe next symbol. Each of the reset and read stages takes about 25% ofone symbol time in this example, leaving 50% for evaluation.

Input signal Data is a PAM-4 signal (for pulse-amplitude modulation,4-level) in the example of FIG. 1. That is, signal Data is modulatedusing four pulse amplitudes to represent symbols, each symbol conveyingtwo binary bits (i.e., 00, 01, 10, and 11). Quantizer 120[16:1]quantizes each analog level each symbol of sixteen 3.5 GBd seriesDa[16:1](k) into two bits for a total data rate of 112 Gb/s (3.5GBd*16*2 bits=112 Gb/s). Binary or other modulation schemes can be usedin other embodiments.

In the foregoing description and in the accompanying drawings, specificterminology and drawing symbols have been set forth to provide athorough understanding. In some instances, the terminology and symbolsmay imply specific details that are not required. The term “coupled,”for example, is used herein to express a direct connection as well as aconnection through one or more intervening circuits or structures.Furthermore, while the subject matter has been described in connectionwith specific embodiments, other embodiments are also envisioned.Therefore, the spirit and scope of the appended claims should not belimited to the foregoing description. Only those claims specificallyreciting “means for” or “step for” should be construed in the mannerrequired under the sixth paragraph of 35 U.S.C. § 112.

What is claimed is:
 1. A receiver comprising: a continuous-timeequalizer to equalize a continuous-time signal conveyed at a symbolrate, thereby providing an equalized continuous-time signal at thesymbol rate; analog samplers each coupled to an output of thecontinuous-time equalizer, the analog samplers to sample the equalizedcontinuous-time signal, thereby providing multiple series of analogsamples, each of the multiple series of analog samples conveyed at asample rate lower than the symbol rate; and for each of the analogsamplers: a discrete-time variable-gain amplifier coupled to an outputof the analog sampler to amplify the respective series of analogsamples, thereby providing an amplified series of analog samples; and aquantizer coupled to an output of the discrete-time variable-gainamplifier to quantize the respective amplified series of analog samples.2. The receiver of claim 1, each of the discrete-time variable-gainamplifiers including: a transistor having: a control terminal coupled tothe output of the analog sampler; a first current-handling terminal; anda second current-handling terminal; and a first switching element toselectively connect the first current-handling terminal to quantizer. 3.The receiver of claim 2, each of the discrete-tire variable-gainamplifiers further including a capacitor connected between the switchingelement and a power-supply terminal.
 4. The receiver of claim 3, each ofthe discrete-time variable-gain amplifiers further including a secondswitching element connected in parallel with the capacitor, the secondswitching element to selectively discharge the capacitor.
 5. Thereceiver of claim 4, further comprising timing circuitry coupled to thediscrete-time variable-gain amplifier, the timing circuitry to controlthe first switching element to charge the capacitor responsive to eachof the analog samples.
 6. The receiver of claim 5, the timing circuitryto control the second switching element to discharge the capacitorbetween the analog samples.
 7. The receiver of claim 6, the secondswitching element to discharge the capacitor to zero volts.
 8. Thereceiver of claim 2, each of the discrete-time variable-gain amplifiersfurther including a current source coupled between the secondcurrent-handling terminal and a power-supply terminal.
 9. The receiverof claim 8, further comprising a second current source coupled betweenthe second current-handing terminal and the power-supply terminal, thereceiver further comprising an impedance disposed between the secondcurrent source and the second current-handling terminal.
 10. Thereceiver of claim 9, wherein the second impedance is programmable.
 11. Amethod for quantizing a continuous-time signal conveyed at a symbolrate, the method comprising: equalizing a continuous-time signal,conveyed at a symbol rate, to provide an equalized continuous-timesignal at the symbol rate; periodically sampling the equalizedcontinuous-time signal responsive to phase-offset clock signals, therebyproviding multiple series of analog samples; amplifying each of themultiple series of analog samples to produces multiple amplified seriesof analog samples; and quantizing each of the multiple amplified seriesof analog samples.
 12. The method of claim 11, wherein the periodicallysampling comprises charging a capacitor responsive to the equalizedcontinuous-time signal.
 13. The method of claim 12, further comprisingdischarging the capacitor between the series of analog samples.
 14. Themethod of claim 13, wherein the discharging of the capacitor sets avoltage across the capacitor to zero.
 15. The method of claim 12,wherein the charging of the capacitor comprises drawing a constantcurrent through the capacitor.
 16. The method of claim 11, wherein theequalized continuous-time signal comprises complementary signal halves.17. The method of claim 11, wherein quantizing each of the multipleamplified series of analog samples comprises quantizing each analogsymbol in each of the multiple amplified series of analog symbols. 18.The method of claim 17, wherein the quantizing produces two binary bitsfor each of the analog symbols in each of the multiple amplified seriesof analog symbols.
 19. A receiver comprising: a continuous-timeequalizer to equalize a continuous-time signal conveyed at a symbolrate, thereby providing an equalized continuous-time signal at thesymbol rate; differential analog samplers each coupled to an output ofthe continuous-time equalizer, the differential analog samplers tosample the equalized continuous-time signal, thereby providing multipleseries of analog samples, each of the multiple series of analog samplesconveyed at a sample rate lower than the symbol rate; and for each ofthe analog samplers, a discrete-time variable-gain amplifier coupled toan output of the analog sampler to amplify the respective series ofanalog samples, thereby providing an amplified series of analog samples.20. The receiver of claim 19, further comprising a quantizer coupled toan output of the discrete-time variable-gain amplifier to quantize therespective amplified series of analog samples.